In the reforming of naphtha to produce high quality gasoline substantial amounts of mixed aromatics are produced, particularly benzene-toluene-xylenes and ethylbenzene. An approach which has been tried for the production of aromatics, primarily a substantially pure stream of benzene from naphtha, is set forth in British Pat. No. 1,157,114. This patent discloses the integration of a reforming unit and a thermal hydrocracking unit for producing predominantly benzene and a fuel gas from naphtha. In a first stage endothermic reaction, a naphtha and hydrogen stream are reacted at about 500 psig in a catalytic reforming operation to produce aromatic compounds from paraffins, olefins and cyclic hydrocarbons. In a second stage exothermic reaction at about the same pressure, the reformate stream is subjected to thermal hydrocracking to produce methane and ethane from paraffins, and to dealkylate aromatic compounds. Steam and/or sulfur is added to prevent coking in the thermal hydrogenation reactor. The principal product stream of this process is a mixture of about 75% benzene, 14% toluene and 7.5% xylene which is then further treated to produce substantially pure benzene, a toluene-rich stream of benzene-toluene-xylene, and a xylene-rich stream containing higher aromatics. The second major product stream is a fuel gas rich in methane, ethane, hydrogen, carbon monoxide and carbon dioxide, and containing sulfur compounds. Although a hydrogen recycle stream may be used, an outside source of hydrogen is required to carry out the processes of this patent.
It is an object of this invention to provide an improved process for producing aromatics from naphtha. For example, it would be desirable to obtain a more efficient economical conversion of naphtha to aromatics such as benzene, toluene, and xylenes and to do so without an outside source of hydrogen.
It also would be desirable to upgrade in value the products of the foregoing process by increasing the xylene, and particularly the paraxylene content for use as a feedstock for the production of terephthalic acid or dimethylterephthalate. Furthermore, it is desirable to increase the available ethane, which as a feedstock for the production of ethylene has a higher value than methane.
Of the normal amount of mixed benzene-toluene-xylenes production, the minor proportion of the xylenes are recovered, for use as solvents and for chemical uses such as the production of terephthalic acid from paraxylene. The C.sub.8 aromatics found in catalytic reformate generally occur in the following ratio
______________________________________ Typical % Range % ______________________________________ Orthoxylene 23 23-26 Metaxylene 40 35-40 Paraxylene 17 16-20 Ethylbenzene 20 17-21 ______________________________________
The total amount of these aromatics in the reformate can be about 15 to 18%, although the amount can vary significantly depending upon the character of the feedstock and the processing conditions. However, even when a catalytic reformer is operated under conditions to maximize the production of benzene-toluene-xylenes further substantial efforts and additional costs are necessary to separate the components of this mixture and to produce a high-purity paraxylene. The paraxylene separation is usually carried out by crystallization or adsorption and the higher the content of paraxylene, the higher the value of the C.sub.8 aromatic fraction. It is therefore preferable to seek first to increase the ratio and amount of xylenes in the product stream by chemical processing steps.
It is therefore a further object of this invention to produce a substantial yield of high purity paraxylene from a relatively wide cut, low quality naphtha. In addition, it is desirable to produce in this process nitration grade benzene, pipe line quality gas high in ethane, propane, and small quantities of higher boiling paraffin compounds.
Another object of this invention is to carry out hydrodealkylation of the effluent from a catalytic reformer operated on high end point naphthas so that the effluent contains C.sub.9, C.sub.10 and C.sub.11 aromatics, making it possible to:
increase the total aromatic content of the effluent to produce and recover as much as 62%, or more, aromatics from an Arabian naphtha; PA1 minimize the formation of methane in the gas produced, and maximizing the ethane content; PA1 hydrocrack substantially all of the paraffins in the effluent to methane, ethane, and propane without the need for hydrogen beyond that produced in the reformer; PA1 produce a mixed xylene fraction with an enhanced paraxylene content; and PA1 accomplish the foregoing through processing techniques that reduce investment and operating costs substantially below conventional methods. PA1 Other objects will be set forth in the following detailed description of the invention.